Gemfile

source "https://rubygems.org"

gem "whatever"

# Load minitest and red-green
group :minitest do
  gem "minitest"
  gem "minitest-rg"
end

run_tests.rb

#!/usr/bin/env ruby
# Put this in bin/run_tests.rb or something and chmod 755 it

# Load bundler
require "bundler/setup"
Bundler.require :default, :minitest

# Load your code
require_relative "../lib/my_gem.rb"

# Run minitest with ARGV
Dir["tests/**/*.rb"].each &method(:load)
Minitest.run ARGV

The key learning before this setup works is twofold:

1. If your test suite is fast, you generally want to run the whole test suite all the time. Sometimes you need to focus on a single test, but you can do that with the arguments you pass into Minitest.run
2. You don't actually want any test setup, support, or environment code. This means to test/test_helper.rb, no test/support/*, etc. If you need sample data, stick it in a directory called sample_data and write a loader class for it. Bundle that loader class in with your library, it'll actually help people actually exercise your code.

Writing test suites like this, with no fluff around them, helps the TDD process by forcing you to feel maximum pain when your objects are difficult to demonstrate via exercising simple examples.